A common problem in designing fiber-optic lighting systems is to minimize the size (i.e., diameter) and number of light guides required to deliver sufficient light. The smaller the fibers required, the lower the cost and the easier to install. Smaller fibers are more flexible and more easily concealed. If multiple outputs are required, the least expensive and neatest method is to run one large fiber from a light source to an intermediate position and then split the light into a number of smaller fibers. If the large fiber is transporting light at angles up to its acceptance angle, then the coupler should make the split without substantially increasing the angular distribution of light, to reduce light loss. This is possible in principle, if the total areas at input and output are the same, which is often not practical due to design issues, owing to the law of conservation of etendue (or brightness or sparkle of light). The law only states that it is possible, but does not require it.
A primary goal in designing a coupler is to avoid light loss in the coupler. In an ideal case, the aggregate area of the outputs can be the same as the area of the input where the numerical apertures of the input and outputs are the same. Since this is not always practical due to fabrication issues, it is often necessary to allow the aggregate area of the outputs to increase to approximately a factor of two over the input area.
A further goal is to avoid light loss in coupling light to, for instance, output light guides. For this it is necessary that the angular distribution of light at the output not exceed the acceptance angle of the output light guides. Where the numerical aperture of the input and output fibers are the same, this is accomplished by not substantially increasing the angular distribution of light during splitting.
A yet further goal of some embodiments is to split the light between the output ports in a controlled manner. The most typical example is splitting the light evenly between the different ports. It is most often desired, if not a requirement of a fiber system, that the fraction of light and the color of the light be the same for all the ports so that post installation testing can be minimized.
Another goal is to provide output ports which are spatially separate from each other. In the case of typical large core plastic optical fibers, this eliminates the need to strip the cladding and jacket off of the fiber. In the case where the coupler is used as a combiner, this allows space for mechanical packaging of the input sources.
A yet further goal is to minimize the area of the output, since smaller-sized fibers are more economical, etc. It is also desirable to preserve the etendue of the coupled light to a high degree. Preserving the etendue is especially important since it enables the size of the optics that are attached to the fibers, such as automotive headlamps, to be minimized.